The present invention relates to novel methods for producing fusion cast basic refractory materials, particularly those refractories produced from magnesia and chrome ore. In particular, this invention relates to such refractories which are generally used in the metallurgical industry to line or reline ferrous and non-ferrous processing vessels.
It has long been known to make various types of refractory articles. Natural refractory articles are made from natural clays or other materials. Artificial refractory articles may be made of material which is pressed or sintered, chemically bonded or formed by fusion in an electric or other furnace. Artificial refractory articles are often formed of materials such as fused alumina, magnesia, mullite (alumina silicate), magnesium aluminate, and silicon carbide.
Cast refractory articles are regularly formed by fusing the desired constituents in an electric furnace, pouring the fused molten material into a mold, and then cooling the casting in such a way as to prevent it from cracking. The refractory article may be cast into its preferred shape, or into a billet which is cut and/or ground into its desired shape. Alternatively, the refractory material, before or after initial use, can be crushed into grain which is pressed, with additions of other materials, such as binders, into shapes which are fired (sintered) to form rebonded fused grain refractory materials.
U.S. Pat. Nos. 1,615,750 (Fulcher); 2,690,974 (Magri, Jr.); and 2,599,566 (Magri, Jr.); 2,911,313 (Sandmeyer); 3,079,452 (Grollier-Baron et al.); 3,116,156 (Scharvat); 3,132,954 (Alper et al.); 3,198,643 (Alper et al.); 3,337,354 (Alper et al.); 3,763,302 (Duchenoy et al); and 4,107,255 (Clishem et al.), each disclose various refractory materials and the methods which may be used for making them.
As seen from the aforementioned patents, an advantageous refractory exhibits good resistance to corrosion and erosion under operating conditions, good resistance to spalling or cracking in response to thermal cycling; high levels of flexural rupture strength (as indicated by the modulus of rupture or MOR), minimal porosities and handling characteristics which permit casting, sawing, grinding, and rebonding by sintering (with or without other constituents) into suitable refractory materials.
Refractory materials composed of certain mineral oxides or mixtures of oxides, such as silica, alumina, zirconia, chrome oxide, magnesia, alkaline oxides, etc. are often used in the glass industry in order to produce linings for various apparatus. In the manufacture of glass refractories, molten refractory material, typically melted in electric arc furnaces operating at temperatures in the range of 1800.degree. C., are melted, highly oxidized and/or oxygen lanced in their molten state. The result is a highly oxidized "white product". These white products are normally cast in their final shapes.
It is also known that various dense fused heat-cast refractories may be made from chrome ore and magnesia. Such products are disclosed in aforementioned U.S. Pat. Nos. 1,615,750, 2,690,974, and 2,599,566. As disclosed in U.S. Pat. No. 2,690,974, such refractories contain FeO, MgO, Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3, and are particularly characterized by their periclase and spinel phases. Such chrome-magnesia refractories are particularly suited for use in the manufacture of steel, which may consume several pounds of basic refractories for each ton of steel produced.
In the production of such fused cast metallurgical refractories, chrome ore, magnesite, and a variety of other materials are usually melted in electrical arc furnaces which comprise carbon/graphite electrodes. It is well known that the use of such electrodes in this process results in certain changes in the chemical composition of the melt. For example, U.S. Pat. No. 2,599,566 teaches that the use of graphite electrodes results in reduction of some of the FeO and the Cr.sub.2 O.sub.3 as well as volatization of some of the MgO. According to this '566 patent, the percentages of Cr.sub.2 O.sub.3 and FeO in the fused product are substantially those of the batch. On the other hand, increases in alumina occur at the expense of magnesia, and should be compensated for through the addition of extra amounts of calcined magnesite to the batch.
U.S. Pat. No. 3,079,452 discloses that the arcs between carbon electrodes have a marked reducing action when they are short, while when they are long the carbon given off by the electrodes is burnt in its course so that the reducing action becomes weaker, at least when the arc strikes in an oxidizing atmosphere, as, for example, free air. This patent points out that it could not be foreseen that the "particular faults of bubbling and cracking encountered with refractory products based on mineral oxides were due to a reducing action, particularly that of carbon, because in the glass industry, for example, the phenomenon of bubbling can have many causes". Accordingly, the '452 patent teaches the suppression or weakening of these defects by the use of a sufficiently long arc between the charge and an electrode or electrodes, so that the carbon given off by the electrodes might have time to burn before reaching the bath. To achieve this aim, the '452 patent discloses the desirability of minimizing the reducing action from the electrode by lengthening the arc created thereby, while at the same time agitating the molten product to improve its porosity. The '452 patent states:
"In this way applicants have observed that with a short arc, such as produced at the instant of striking of the arc following the moment when an electrode is moved away from the bath, the surface of the bath remains calm, while as one lengthens the arc by progressively increasing the distance of the electrode from the bath and if the intensity of the current is suitably adjusted there comes a moment when an intense agitation is visible at the surface of the bath. This agitation and the corresponding length of the arc often coincide, moreover, with the phenomenon of a `whistling` arc".
The operation of electrical arc furnaces with "whistling arcs" is now the accepted standard in the glass refractory industry. In the production of metallurgical refractories similar electrode spacings are also the norm. When metallurgical refractory batches are processed under these conditions loud "arc and bark" melt conditions prevail. The "arc and bark" sound is a non-continuous, electrical discharge sound with random amplitude and random frequency occurences. The pitch and intensity of this sound vary over a wide range, from a sharp crackling noise to an abrupt and/or sustained roar.
It is also understood that the nature of the gaseous atmosphere in the fusion furnaces above the mass of molten matter plays an important part in the quality of products obtained, and that it is generally desirable to renew this atmosphere to ensure that it remains neutral or oxidizing. The circulation of air across the surface of the molten mass is now routinely accomplished using environmentally mandated dust collection equipment which draws air across a surface of the molten material which is in a state of constant agitation caused by whistling arcs.
It is further known that molten refractory materials may be "grogged" through additions of certain solid particulate materials which are principally added for a variety of purposes. One of these is to improve the spalling or cracking resistance of the resultant cast refractory. A variety of grog materials have been suggested for this purpose, including, for example, crushed solid particles of the cast material itself, or of various components of the batch from which it was made. In this regard, please refer to U.S. Pat. No. 4,107,255, which describes that grogging may improve the modulus of rupture at temperatures in the range of 1340.degree.-1500.degree. C. of a refractory cast from a molten mass having a composition consisting essentially (by weight) of 45-78% MgO, 0-30% Cr.sub.2 O.sub.3, 0-35% Al.sub.2 O.sub.3, 0-17% FeO+Fe.sub.2 O.sub.3, at least 82% MgO+Cr.sub.2 O.sub.3 +Al.sub.2 O.sub.3 +FeO plus Fe.sub.2 O.sub.3, 1-8% SiO.sub.2, 0-2% CaO+BaO plus SrO.sub.2, 0-10% TiO.sub.2 and 0-3% fluorine. In this patent, non-fused oxidic inorganic grog particles having losses on ignitions (L.O.I.) at 1000.degree. C. of less than about 0.5% by weight and an aggregate SiO.sub.2 content at least about equal to the SiO.sub.2 content of the molten mass forming the cast refractory, are grogged into the molten material to effect the described improvement. A product made in accordance with the teachings of this patent is currently available under the tradename ENDUR.
Notwithstanding the improvements which have been made in the production of magnesia-chrome fusion cast refractories, such refractories remain subject to corrosion, erosion, cracking, spalling and other failures when used as linings of ferrous or non-ferrous production vessels. For this reason, the use of such refractories has been displaced to some extent by generally more expensive magnesia-carbon bonded refractories. Accordingly, a need exists for an improved, lower cost fusion cast refractory which exhibits improved corrosion-erosion resistance, and suitable thermal cycling, spalling and cracking resistance characteristics.